Unveiling the mechanism of N‐methylation of indole with dimethylcarbonate using either DABCO or DBU as catalyst
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Depending on the catalyst used, N‐methylation of indole with dimethylcarbonate (DMC) ‐ an environmentally friendly alkylation agent – yields different products. With 1,4‐diazabicyclo[2.2.2]octane (DABCO), the reaction forms only N‐methylated indole but with or 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), both N‐methylated and N‐methoxycarbonylated indole are formed. Using direct ESI(+)‐MS monitoring to collect actual snapshots of the changing ionic composition of the reaction solution, we report on the interception and characterization of key intermediates for such reactions. Although a mechanism has been proposed with methoxycarbonylated base as the key intermediate for both DBU and DABCO, the ESI(+)‐MS data as well as B3LYP‐D3/6‐311+G** calculations suggest that the reaction of DMC with indole under either DABCO or DBU catalysis follow contrasting mechanisms.
Vendramini , P H , Zeoly , L A , Cormanich , R A , Buehl , M , Eberlin , M N & Ferreira , B R V 2021 , ' Unveiling the mechanism of N ‐methylation of indole with dimethylcarbonate using either DABCO or DBU as catalyst ' , Journal of Mass Spectrometry , vol. 56 , no. 3 , e4707 . https://doi.org/10.1002/jms.4707
Journal of Mass Spectrometry
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DescriptionB.R.V.F., P.H.V. and M.N.E. acknowledge the São Paulo State Science Foundation (FAPESP) and the Brazilian National Science Council (CNPq) for financial support. L.A.Z. thanks CNPQ for his PhD scholarship (142476/2018-8), R.A.C. thanks SDumont and CESUP. M.B. thanks the School of Chemistry and EaStCHEM for support and for access to a computer cluster maintained by Dr. H. Früchtl.
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